Gas mileage #'s

[turtlevette]

It's possible to get a carbureted small block to run similar numbers, but you do need to play with the IFR sizing and low speed air bleeds in order to get the cruise mixture optimized for mileage: Primary jetting has little effect on cruise mixture, since most fuel at cruise is metered through the transition circuit, which is controlled by the IFR sizing and not the main metering jets. If you try to jet a carb down enough to lean out the cruise mixture to optimum, you tend to get severe stumbles and poor WOT performance. For this reason, few carbs are really mpg optimized for their specific application, making it appear that TPI is the only way to squeeze the mileage out of these cars. With a bit of work and a good A/F monitor, you can get very good cruise mileage numbers from a carbureted small block.

Lars

I don't remember what IFR means. Your saying the idle screws determine the cruise mixture?

I thought it was impossible to get the cylinder to cylinder tuning the same since you can't control how the mixure disperses so you have to run much richer with a carb. Do you run with the exhaust crossover to better vaporize the fuel? I guess not. Sounds like you don't run a carbed engine.

 

[lars]

IFR is the Idle Fuel Restrictor, and it is the primary metering for fuel at cruise - most carbs run on the fuel metered through the transition slots and idle mixture screws at cruise because the throttle blade opening is not big enough at cruise, nor is air mass suffucient, to pull fuel out of the main discharge nozzles at light throttle settings. Proper tuning of the IFRs and low speed air bleeds can yield cruise fuel economy on a carb very close to TPI on a mildly built motor. The IFRs meter the fuel prior to both the idle mixture screws and the transition slots, and is the only fuel metering for the transition slots. At light cruise and high manifold vacuum settings, fuel is pulled additively from both the idle mixture discharge ports and from the transition slots. Since both of these discharge locations are in the throttle bore - and not in the venturi of the carb - fuel is metered from these locations when the throttle is only partially open and when manifold vacuum can "pull" the fuel from these locations (such as at light cruise). At moderate power settings and WOT, fuel is dicharged from the main discharge nozzles, and very little fuel is pulled through the IFR circuits.

I don't run a carbed engine? The engine referenced in my initial post response is a TPI engine as stated in my post: "I drive my TPI 350 Vette 110 miles a day." I also have 5 cars with carbs and another 3 with TPI.

Lars

 

[Ozzy]

A lot of these claims just don't add up. The brakes on these things drag like crazy. The coefficient of drag is not good. I'm sure there are other intrinsic reasons why these cars will never achieve some of the numbers being claimed here. You can't change the laws of physics. If you have a big block with carb and you're getting 17 mpg in town, you better check your math! I think most of you need to check your math.

My brand new mustang V6 gets around 25-28 on the hwy. There's no way guys....your getting these kinds of numbers.

I had a 283 in mine at one time and drove back and forth to work on back roads at approx 45mph which is about the most efficient speed for mileage and i got 17 mpg. The carb was running lean too. Oh, yea with the 700r4 and 3.08 gears.

I did not locate the drag cofficient for the chrome bumper C3's,here are a few others to compare:

80 coupe .443
87 coupe .33
02 coupe .29

Stock 1987 A4.L98 C4 coupe, drivers window up or down, varied with the outside temps, Las Vegas to Spokane 69 mph indicated on speedo of Vette and 95 Camaro I was travelling with. 28.6 ave mpg for the trip.

Stock '02 LS1.A4 Coupe, windows up w/ ac running if needed.
Spokane-Vegas-Spokane speeds of legal to 20% above at various times. 26.4 ave mpg round trip.

Both cars get low 16's around town.

 

[AJ77]

Last time I checked mine was last year going to Eureka Springs Arkansas and back from Ft. Worth Tx. I drive the speed limit and have a 383 with aftermarket TPI, a 700R-4 and 3.70 rear gears. I filled up a total of 5 times over the 4 day trip and averaged 22.6 MPG. On the fuel economy run I got 24.6.

 

[DeeVeeEight]

Some of these numbers are awesome and after reading Lars' responses it makes me want to take the Holley off and fine tune it.

:drink:

 

[Bee Jay]

I've got 3.55 gears and a 700r4 trans. I run a Summit 1105 cam, 224/234 duration at .050, AFR 195 heads, and Holley Throttle Body Injection. I drive 200 miles and I fill up with 20 gallons. I have a heavy foot, but 10mpg gets old when it's $5 a gallon. I hope to improve my gas mileage significantly by switching to a milder LT4 Hot Cam hydrualic roller,218.228 duration at .050. Rollers are supposed to have less friction, right? Less friction, less duration, better gas mileage right? My 1500HD Crew Cap with a 6.0 liter LS engine gets 14mph on the road. I should get at least that with my 3200 lb Corvette, right? Maybe I shouldn't give a damn like kwplot.

 

[mrvette]

I've got 3.55 gears and a 700r4 trans. I run a Summit 1105 cam, 224/234 duration at .050, AFR 195 heads, and Holley Throttle Body Injection. I drive 200 miles and I fill up with 20 gallons. I have a heavy foot, but 10mpg gets old when it's $5 a gallon. I hope to improve my gas mileage significantly by switching to a milder LT4 Hot Cam hydrualic roller,218.228 duration at .050. Rollers are supposed to have less friction, right? Less friction, less duration, better gas mileage right? My 1500HD Crew Cap with a 6.0 liter LS engine gets 14mph on the road. I should get at least that with my 3200 lb Corvette, right? Maybe I shouldn't give a damn like kwplot.

All I know is that with my old rebuilt by PO L48 engine, muncie tranny 336 gears, I got 24 mpg on I95 crusing here looking for a house...Feb 97, TPI was installed ..so NO overdrive,

but living here I went '89 truck roller engine and overdrive, and have ruined the fuel economy....lucky to see 20 on the freeways now....

I don't understand it.....the cam is nearly a ZZ4 hot spec.....decent idle went from 650 to 850, I had thought the overdrive would bring back any engine losses....guess not....

:crap:

 

[AJ77]

My cam is somewhat radical. It's a Crane retrofit roller with 112* lobe separation, 222, 236 duration @ .50 and .518", .539" lift with 1.5 Crane roller rockers. The car idles @ 850 rpm and is jumping up and down. I need to find time to get my tuner to take a ride with me so he can tweek the system with his laptop. I think I might be running a little lean. I think my mileage has dropped since I had the tranny rebuilt. It shifts at higher speeds now so that will eat up some mileage. Oh well, if I have to worry about that, I don't need the car--right?

 

[4 SPEED]

I have been averaging 18-20 per gal with windows up or down or tops on or off.
I have a 700dp Holley, M20 4 speed and a 3:08 rear.
Total hp around 400

When I get heavy on the pedal forget about it.


Schoon

 

[mrvette]

I have been averaging 18-20 per gal with windows up or down or tops on or off.
I have a 700dp Holley, M20 4 speed and a 3:08 rear.
Total hp around 400

When I get heavy on the pedal forget about it.


Schoon

A HOLLEY and no overdrive, and you getting same as me, 10% more gear in my car, but overdrive, .70.....got FI, closed loop even....

my cam sux gas and ass.....

:goodnight::flash:

 

[DARROW 74]

454 Stock

About 9 Seems About With Air On.

 

[68/70Vette]

I had a L88 engine short block with the cast iron head versions of the aluminum heads. The compression ratio was originally 12.5:1. I had the piston domes machined down to arrive at a 11.5:1 compression ratio. The car had a 3.08:1 rear end differential. The engine was in a 1969 Coupe and I had Hooker header sidepipe exhausts; i.e. open exhausts. With a full tank, 20 gallons, I could drive to Las Vegas (300 miles) with about 1/4 tank of fuel remaining. My engine guy estimated that by lowering my compression ratio one point , I lost 50 hp. So 550 hp - 50 hp, meant that the car was getting approx 20 mph/gallon with a 500 hp engine. Of course, with a 3.08 rear end at 70 mph, my engine was turning very low rpm and correspondingly producingly very low hp at that rpm rating.

 

[Bee Jay]

I've got 3.55 gears and a 700r4 trans. I run a Summit 1105 cam, 224/234 duration at .050, AFR 195 heads, and Holley Throttle Body Injection. I drive 200 miles and I fill up with 20 gallons. I have a heavy foot, but 10mpg gets old when it's $5 a gallon. I hope to improve my gas mileage significantly by switching to a milder LT4 Hot Cam hydrualic roller,218.228 duration at .050. Rollers are supposed to have less friction, right? Less friction, less duration, better gas mileage right? My 1500HD Crew Cap with a 6.0 liter LS engine gets 14mph on the road. I should get at least that with my 3200 lb Corvette, right? Maybe I shouldn't give a damn like kwplot.

11.04mpg on my last tank. Damn.
Bee Jay

 

[67-427ci]

I just returned from a 9,200 mile 37 day cross country cruise in my 67 Coupe. Average mileage was 17.6 MPG (high of 19.8 & low of 15.5 depending on terrain). My engine is a zz454 440 HP big block with a Tremec TKO-600 (.64 OD) and 3.70 rear end. 75 MPH is 2,200 RPM. With the old M21 my mileage was around 12.
Roy

 

[greg75vette]

Last time I took mine out for a long cruise, I went 170 miles on 10 gallons of gas. Not bad for an original drivetrain. '75 L48 (165hp) 4speed with, I believe, 3.54 rear.

 

[mrvette]

I just returned from a 9,200 mile 37 day cross country cruise in my 67 Coupe. Average mileage was 17.6 MPG (high of 19.8 & low of 15.5 depending on terrain). My engine is a zz454 440 HP big block with a Tremec TKO-600 (.64 OD) and 3.70 rear end. 75 MPH is 2,200 RPM. With the old M21 my mileage was around 12.
Roy

:shocking:

that's GREAT MPG there, hope your trip was as fine...looks like FUN...

:1st: start a thread with some pix....:beer:

 

[gkull]

I've got 3.55 gears and a 700r4 trans. I run a Summit 1105 cam, 224/234 duration at .050, AFR 195 heads, and Holley Throttle Body Injection. I drive 200 miles and I fill up with 20 gallons. I have a heavy foot, but 10mpg gets old when it's $5 a gallon. I hope to improve my gas mileage significantly by switching to a milder LT4 Hot Cam hydrualic roller,218.228 duration at .050. Rollers are supposed to have less friction, right? Less friction, less duration, better gas mileage right?

11.04mpg on my last tank. Damn.
Bee Jay

To get the best MPG is not just lowering the rpm. You get the best MPG where the motor is operating efficiently. Crane cams used to print the operating range on cams. If it said 2800 - 3400 rpm that is where you are going to get the best MPG.

So BJ my 700R4 equipped Vette worked best with a 4.11 rear end to get the rpm for efficiency up for my typical freeway speed. Actually 3.90 would have been the choice.

Your TQ converter could be robbing you of a couple of MPG. Are you running in lockup OD?.

Even my present motor gets over a 14 average just screwing off in town the whole 25 gallons. My last tank was 386 miles and 23.6 gallons

 

[redvetracr]

4mpg......

 

[big2bird]

4mpg......

Yeah, but you get more "smiles per gallon" than anyone here.:1st:

 

[brngrhd]

i don't even know LOL my combo is not setup for MPG it is sep up for fun.... 3000stall and a healthy big block the only thing i got going is the 3.25 gears.....

but as gkull said it is not about rpm it is about the whole combo the cam gearing and load people with big cams and low rpm will not be a good thing... the motor is just not efficient there......

 

[bobs77vet]

well with my mildly built 355 i get about 12 in the city (washington dc area)....now if you can get me out on an interstate with the 700r4 od and the 3.08 i get 23-26 at 65-70